The present invention, relates generally to power supplies, and, more specifically, to output current control in a boundary conduction mode converter by sensing current in its controlled switch.
Current-programmed control, a scheme in which the output of a switch-mode power supply (SMPS) is controlled by choice of the peak current in a controlled switch, finds wide applications due to its ease of implementation, fast transient response and inherent stability. The peak current in the controlled switch is representative of the average current in inductive elements offset a ripple current amplitude. Ideally, in a boundary conduction mode converter, the average current in its inductive element equals one-half of the peak current. However, due to parasitic elements of the circuit, such as parasitic capacitance of switching and inductive elements, reverse recovery delays of rectifier diodes, controlling the peak current produces an error with respect to the average output current. This error affects the accuracy of the current control loop and diminishes the benefits of the control method.
Due to the above issues, circuits and methods have been designed which eliminate the peak-to-average current sense error in a current-programmed control (CPC) circuit of a boundary conduction mode switching converter. The switching converter receives energy from an input voltage source and delivers this energy to the output load by storing it fully or partially in one or more inductive elements. The energy is directed by periodical switching of two or more switching devices, at least one of which devices being controlled switches. In CPC, the conduction time of the controlled switch is determined by the time required for the current in the inductive element to reach a programmed level. However, in these circuits and methods, an error is contributed by a negative swing of the current in the inductor.
Therefore, it would be desirable to provide a system and method that overcomes the above problems.